Finding and querying documents in Express - Mini Project: Build & Apply

Finding and Querying Documents with Express and MongoDB

📖 Scenario: You are building a simple Express server that connects to a MongoDB database. The database stores a collection of books with their titles and authors.You want to create routes to find and query these book documents.

🎯 Goal: Build an Express server with a MongoDB connection that can find all books and query books by author.

📋 What You'll Learn

Create a MongoDB collection named books with sample data

Set up an Express server with MongoDB connection

Create a route to find all books

Create a route to query books by author using a query parameter

💡 Why This Matters

🌍 Real World

Web servers often need to find and filter data from databases to show users relevant information.

💼 Career

Backend developers use Express and databases like MongoDB to build APIs that find and query documents efficiently.

1

Set up the books collection with sample data

Create a constant called books that is an array of objects with these exact entries: { title: 'The Hobbit', author: 'J.R.R. Tolkien' }, { title: '1984', author: 'George Orwell' }, and { title: 'To Kill a Mockingbird', author: 'Harper Lee' }.

Express

// Create the books array with the given objects
// Your code here

Hint

Use const books = [ ... ] with the exact objects inside the array.

2

Set up Express server and MongoDB connection

Create a constant called express by requiring 'express'. Then create a constant called app by calling express(). Also create a constant called port and set it to 3000.

Express

const books = [
  { title: 'The Hobbit', author: 'J.R.R. Tolkien' },
  { title: '1984', author: 'George Orwell' },
  { title: 'To Kill a Mockingbird', author: 'Harper Lee' }
];

// Set up Express server and port
// Your code here

Hint

Use const express = require('express'), then const app = express(), and set const port = 3000.

3

Create route to find all books

Use app.get with the path '/books' and a callback with parameters req and res. Inside the callback, send the books array as JSON using res.json(books).

Express

const books = [
  { title: 'The Hobbit', author: 'J.R.R. Tolkien' },
  { title: '1984', author: 'George Orwell' },
  { title: 'To Kill a Mockingbird', author: 'Harper Lee' }
];

const express = require('express');
const app = express();
const port = 3000;

// Create GET route for '/books' to send all books
// Your code here

Hint

Use app.get('/books', (req, res) => { res.json(books); }) to send all books.

4

Create route to query books by author

Use app.get with the path '/books/search' and a callback with req and res. Inside, get the author query parameter from req.query.author. Filter the books array to only include books where book.author exactly matches the author query. Send the filtered array as JSON with res.json(filteredBooks). Finally, start the server listening on port with app.listen(port).

Express

const books = [
  { title: 'The Hobbit', author: 'J.R.R. Tolkien' },
  { title: '1984', author: 'George Orwell' },
  { title: 'To Kill a Mockingbird', author: 'Harper Lee' }
];

const express = require('express');
const app = express();
const port = 3000;

app.get('/books', (req, res) => {
  res.json(books);
});

// Create GET route '/books/search' to filter books by author query
// Start server listening on port
// Your code here

Hint

Use app.get('/books/search', (req, res) => { ... }), get author from req.query.author, filter books by exact author match, send filtered books with res.json, and start server with app.listen(port).

Practice

1. In Express with Mongoose, which method is used to find all documents matching a condition in a collection?

easy

A. Model.delete()

B. Model.create()

C. Model.update()

D. Model.find()

Solution

1. Step 1: Understand the purpose of Model.find()

   This method retrieves documents from the database that match the given query conditions.

2. Step 2: Differentiate from other methods

   Model.create() adds new documents, Model.update() modifies existing ones, and Model.delete() removes documents.

3. Final Answer:

   Model.find() -> Option D

4. Quick Check:

   Find documents = Model.find() [OK]

Hint: Use find() to get matching documents [OK]

Common Mistakes:

• Confusing find() with create()
• Using update() to retrieve documents
• Trying to delete documents to find them

2. Which of the following is the correct syntax to find all users with age 30 using Mongoose in Express?

easy

A. User.find({ age: 30 })

B. User.find(age == 30)

C. User.find('age: 30')

D. User.find({ 'age' = 30 })

Solution

1. Step 1: Use an object to specify query conditions

   The query must be an object with key-value pairs, like { age: 30 }.

2. Step 2: Check syntax correctness

   User.find({ age: 30 }) uses correct JavaScript object syntax. Options B, C, and D have syntax errors or wrong formats.

3. Final Answer:

   User.find({ age: 30 }) -> Option A

4. Quick Check:

   Query object syntax = User.find({ age: 30 }) [OK]

Hint: Use curly braces with key: value for queries [OK]

Common Mistakes:

• Using comparison operators inside find()
• Passing query as a string
• Using assignment (=) instead of colon (:) in object

3. What will be the output of this code snippet in Express using Mongoose?

const users = await User.find({ active: true }, 'name email');
console.log(users);

medium

A. An array of user objects with only 'name' and 'email' fields

B. An array of user objects with all fields including 'active'

C. A single user object with 'name' and 'email' fields

D. An error because projection syntax is wrong

Solution

1. Step 1: Understand the query parameters

   The first argument is the filter { active: true }, so only active users are returned.

2. Step 2: Understand the projection string

   The second argument 'name email' selects only these fields to be included in the returned documents.

3. Final Answer:

   An array of user objects with only 'name' and 'email' fields -> Option A

4. Quick Check:

   Projection limits fields = An array of user objects with only 'name' and 'email' fields [OK]

Hint: Second argument in find() selects fields to return [OK]

Common Mistakes:

• Expecting all fields to be returned
• Thinking projection returns a single object
• Using wrong projection syntax causing errors

4. Identify the error in this Mongoose query:

const results = await Product.find({ price: { $gt: 100 } }, { name, price });

medium

A. The find method cannot use async/await

B. The query operator $gt is invalid

C. The projection object is missing quotes around field names

D. The filter object should be a string

Solution

1. Step 1: Check the projection argument

   The projection should be an object with field names as strings or keys, like { name: 1, price: 1 }.

2. Step 2: Identify the syntax error

   Using { name, price } without values or quotes is invalid JavaScript object syntax here.

3. Final Answer:

   The projection object is missing quotes around field names -> Option C

4. Quick Check:

   Projection keys must be strings or key-value pairs [OK]

Hint: Projection keys need explicit values or quotes [OK]

Common Mistakes:

• Using shorthand object keys without values in projection
• Misunderstanding $gt operator
• Thinking async/await is invalid with find()

5. You want to find all orders where the status is 'shipped' and the total is greater than 50, but only return the order ID and total fields. Which is the correct Mongoose query in Express?

hard

A. Order.find({ status: 'shipped', total: { $gt: 50 } }, { _id: 1, total: 1 })

B. Order.find({ status: 'shipped', total: { $gt: 50 } }, '_id total')

C. Order.find({ status: 'shipped', total: { $gt: 50 } }, { _id: true, total: true })

D. Order.find({ status: 'shipped' && total > 50 }, '_id total')

Solution

1. Step 1: Write the correct filter object

   The filter must be { status: 'shipped', total: { $gt: 50 } } to find orders with status 'shipped' and total greater than 50.

2. Step 2: Use correct projection syntax

   Passing a string with space-separated field names like '_id total' is valid for projection in Mongoose.

3. Step 3: Evaluate options

   Order.find({ status: 'shipped', total: { $gt: 50 } }, { _id: 1, total: 1 }) uses an object projection with 1s which is valid but the question asks for the best correct query. Order.find({ status: 'shipped', total: { $gt: 50 } }, '_id total') uses string projection which is simpler and correct. Order.find({ status: 'shipped' && total > 50 }, '_id total') has invalid filter syntax. Order.find({ status: 'shipped', total: { $gt: 50 } }, { _id: true, total: true }) uses boolean true instead of 1 which is invalid in Mongoose projection.

4. Final Answer:

   Order.find({ status: 'shipped', total: { $gt: 50 } }, '_id total') -> Option B

5. Quick Check:

   Filter object + string projection = Order.find({ status: 'shipped', total: { $gt: 50 } }, '_id total') [OK]

Hint: Use object for filter, string for projection fields [OK]

Common Mistakes:

• Using logical operators incorrectly in filter
• Using boolean true instead of 1 in projection
• Passing projection as object with invalid values